1. Field of The Invention:
This invention relates generally to circuitry for monitoring the state of contamination of a reverse osmosis filter used to purify drinking water, and more particularly to a circuit for comparing the concentration of total dissolved solids at the inlet and outlet of such a filter and for providing an indication that the filter is in need of service when the difference drops below a predetermined level.
2. Discussion of the Prior Art:
So-called reverse osmosis water purification systems are finding increased commercial and residential use. Water supplied by many municipalities contain a variety of dissolved (ionized) solids that can affect the flavor and potability of the water.
A typical reverse osmosis filtering system incorporates a cartridge housing containing a semi-permeable membrane and placed in a water supply line downstream of any water softening equipment that may be present. The membrane in question exhibits a propensity for equalizing the concentration of metallic ions dissolved in the water on opposed sides of the membrane. If the concentration of ions in the water is greater on one side than on the other, the water will pass through from the side of lesser ion concentration to the side of greater ion concentration. The force causing the water to move is, of course, the osmotic pressure. By increasing the pressure on the fluid on the side of the membrane exhibiting the greater metallic ion concentration, the flow of water due to osmotic pressure can be reversed. At this point, the membrane functions as a filter for separating the metallic ions from the water in which they are dissolved.
With time, the membrane becomes contaminated in that its pores become plugged with contaminants and the ability of the membrane to separate out dissolved solids drops and replacement of the filter cartridge is required.
It is, of course, advantageous to provide a means for signaling the homeowner or commercial customer that the RO cartridge is in need of replacement. Mere passage of time cannot be relied upon as a good indicator since the condition of the filter membrane can change abruptly, for example, when the municipality may change wells or incorporate an additive into the water supply for one purpose or another.
The Malgren U.S. Pat. No. 3,990,066 describes a RO filter system in which conductivity sensors are disposed on the inlet and outlet sides of the filter cartridge and a Whetstone bridge network is used to detect a change in conductivity of the water flowing at the outlet of the RO filter as compared to that at its inlet. When the bridge reaches a predetermined state of unbalance, a signal is given. Indicator circuits of the type described in the Malgren patent are somewhat unreliable, especially when it is considered that temperature changes of the water affect the conductivity thereof. False alarms tend to be frequent.
The Gales et al U.S. Pat. No. 4,744,895 also discloses an indicator circuit for use with a RO filter and is operative to cause a green or a red LED to be lit, depending upon whether the water is of sufficient purity or not, respectively. The system of that patent employs a single conductivity cell which forms a voltage divider with a variable resistor whose value can be set to correspond to an acceptable water conductivity level. The centerpoint of the voltage divider is then connected through a bistable inverting amplifier exhibiting hysteresis which, in turn, drives the LEDs. Again, taking into account the manner in which water conductivity changes with temperature, the Gales circuit also suffers from erratic control occasioned by temperature swings in the water supply and, further, is not easily tailored to accommodate conductivity changes occasioned by changes made by the municipality as, for example, when changing wells.